The check engine light illuminates whenever there is malfunction in the electronic control circuit of the engine or transmission. Vehicle engine control computers monitor and control engine performance and transmission shifting operation by receiving feedback from various sensors throughout the engine and transmission they control, such things as fuel delivery, air and fuel mixtures, ignition spark, ignition timing, engine speed, also known as (RPM) road speed and load as well as automatic transmission shift points . As the check engine light comes on it can mean many different things, for example: As the engine control computer picks up a problem it can't adjust the computer will turn the check engine light on and store a trouble "code" in the computers on board diagnostic memory. (A.k.a. OBDII) These trouble codes stored help identify possible problem areas, things like sensors, actuators or a miss-firing engine can be retrieved with an on-board diagnostic scanner (preferred method) or a code reader. A "blinking" light is a more severe problem, this is very obvious because the engine is typically running very poorly or the transmission is not shifting properly.

This is where the confusion really starts some will tend to ignore the check engine light because they feel no difference in the way the vehicle is running not understanding that a malfunctioning system or component triggered the light making other systems work even harder. Failure to have it looked at by a professional in a timely manner can lead to other systems malfunctioning and consequently leading to failure and unnecessary repairs. When the computer can no longer compensate and recalibrate for whatever malfunction is occurring, the computer will override the input and go into preset "run on" program allowing the driver to continue to drive and appear that everything is normal except for the light on the dash. Another confusing thing some people are miss-led to think that the check engine light is only for emissions.

Having codes read do not directly tell what's wrong. The only thing a code really indicates is what system or circuit needs to be tested to isolate the actual problem. Each and every code has its own number to match its system and or circuit, sensor(s) or actuators. Every numerical code also has its own test procedure laid out step by step, each step must be performed in absolute sequence skipping steps makes the test useless and leads to miss diagnosed system(s) and hundreds if not thousands of dollars in unnecessary repairs. The testing procedure is the most critical part and can only be done with a scanner or diagnostic computer "code readers" do not have this capability .Tests will include voltage readings, ohm's resistance, and temperature and time values. These tests are very valuable in finding the root cause of failure and should only be done by a competent experienced professional.